Petroleum may be recovered from subterranean, petroleum-containing formations by penetrating the formation with one or more wells and pumping or permitting petroleum to flow to the surface through these wells, only if certain conditions exist in the formation. The petroleum must be present in an adequately high concentration in the formation, and there must be sufficient permeability or interconnected flow channels within the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the fluid. When the formation has natural energy present in the form of an underlying active water drive, gas dissolved in the petroleum which can exert pressure to drive the petroleum to the producing well, or a high pressure gas cap above the petroleum-saturated formation, this natural energy is ordinarily utilized to recover petroleum in what is commonly referred to as primary recovery. When this natural energy source is depleted, or in the instance of those formations which do not contain sufficient natural energy within the formation to allow primary recovery, some form of supplemental recovery process or enhanced recovery process must be applied to the formation in order to extract additional petroleum therefrom. Supplemental recovery is sometimes referred to as secondary recovery or tertiary recovery, although in fact it may be primary, secondary or tertiary in sequence of employment.
The most commonly employed form of supplemental recovery is waterflooding, which comprises injecting water into the subterranean, petroleum-containing formation to displace petroleum toward a producing well from which it is recovered to the surface of the earth. While waterflooding is successful and economical, water does not displace petroleum efficiently, and therefore a substantial amount of petroleum remains in the formation after conclusion of waterflooding. This has been well recognized in the literature pertaining to oil recovery methods, and there are many prior art references suggesting incorporating surface active agents or surfactants in the flood water for the purpose of reducing the interfacial tension between the injected water and the formation petroleum, which accomplishes recovery of a greater portion of the residual oil after primary production than is possible with waterflooding.
Petroleum sulfonate and other organic sulfonates, as well as organic sulfates, have been proposed in many prior art references for surfactant waterflooding. While these materials are inexpensive and effective under ideal conditions, they are very sensitive to formation water salinity, and cannot be used alone in formations containing water whose salinity exceeds about 30,000 parts per million total dissolved solids. Alkylpolyethoxy sulfates and alkylarylpolyethoxy sulfates have also been proposed for surfactant waterflooding in other prior art references. While these materials are effective in high salinity environments, they are not stable at temperatures in excess of about 150.degree. F. due to hydrolysis of the sulfate, and so their use is restricted to high salinity, low temperature formations.
U.S. Pat. Nos. 3,827,497; 3,890,239; and 3,977,471 describe surfactant waterflooding oil recovery processes using alkylpolyalkoxyalkylene sulfonates or alkylarylpolyalkoxyalkylene sulfonates, and recognize that these surfactants are stable at temperatures as high as 240.degree. F. and tolerant of salinities as high as 220,000 parts per million total dissolved solids.
It is well recognized in the literature that surfactant flooding is effective in mobilizing oil as a result of the drastic reduction in interfacial tension between the oil phase and the aqueous surfactant solution. When an aqueous, surfactant-containing fluid contacts oil in a formation, interfaces are formed of interfacial films occupied by surfactant molecules of which the hydrophobic portion is dissolved in the oil phase and the hydrophilic portion is dissolved in the aqueous phase. The physical properties of the interfacial films play a role in oil mobilization during the course of surfactant waterflooding. If rigid interfacial films are formed, oil mobilization through porous media and banking of oil during surfactant flooding is greatly hindered. Interfacial film rigidity can be determined by measuring the relative film pressure of the crude oil/water interface. It is reported in the literature that crude oils from various reservoirs have different interfacial film pressures. Lowering the interfacial film rigidity of a surfactant system should increase the effectiveness of the surfactant fluid to mobilize oil and thereby should improve the overall tertiary oil recovery efficiency of surfactant flooding.
In view of the foregoing discussion, it can be appreciated that there is a substantial need for an additive for an aqueous surfactant fluid to be employed in surfactant waterflooding oil recovery processes, which effectively reduces the rigidity of the interfacial film, provided the cost of the additive is small compared to the improvement in oil recovery effectiveness resulting from employment thereof.